Support passing inline Astro config to getViteConfig() (#10963)

* Support passing inline Astro config to `getViteConfig()`

* Add changeset

.changeset/nervous-waves-shop.md

@@ -0,0 +1,22 @@
+---
+"astro": minor
+---
+
+Adds support for passing an inline Astro configuration object to `getViteConfig()`
+
+If you are using `getViteConfig()` to configure the Vitest test runner, you can now pass a second argument to control how Astro is configured. This makes it possible to configure unit tests with different Astro options when using [Vitest’s workspaces](https://vitest.dev/guide/workspace.html) feature.
+
+```js
+// vitest.config.ts
+import { getViteConfig } from 'astro/config';
+
+export default getViteConfig(
+  /* Vite configuration */
+  { test: {} },
+  /* Astro configuration */
+  {
+    site: 'https://example.com',
+    trailingSlash: 'never',
+  },
+);
+```

packages/astro/src/config/index.ts

@@ -1,12 +1,12 @@
 import type { UserConfig } from 'vite';
-import type { AstroUserConfig } from '../@types/astro.js';
+import type { AstroInlineConfig, AstroUserConfig } from '../@types/astro.js';
 import { Logger } from '../core/logger/core.js';
 
 export function defineConfig(config: AstroUserConfig) {
 	return config;
 }
 
-export function getViteConfig(inlineConfig: UserConfig) {
+export function getViteConfig(inlineConfig: UserConfig, inlineAstroConfig: AstroInlineConfig = {}) {
 	// Return an async Vite config getter which exposes a resolved `mode` and `command`
 	return async ({ mode, command }: { mode: string; command: 'serve' | 'build' }) => {
 		// Vite `command` is `serve | build`, but Astro uses `dev | build`
@@ -34,7 +34,7 @@ export function getViteConfig(inlineConfig: UserConfig) {
 			dest: nodeLogDestination,
 			level: 'info',
 		});
-		const { astroConfig: config } = await resolveConfig({}, cmd);
+		const { astroConfig: config } = await resolveConfig(inlineAstroConfig, cmd);
 		let settings = await createSettings(config, inlineConfig.root);
 		settings = await runHookConfigSetup({ settings, command: cmd, logger });
 		const viteConfig = await createVite(